The present invention relates to a muffler for an internal combustion engine, such as a small air-cooled two-stroke gasoline engine which is suited for use in small, portable working machines such as a bush cutter or a chain saw, and in particular to a muffler which is capable of complying with exhaust gas regulations.
In view of the recent increasing concerns about environmental problems, reductions in the amounts of HC, CO, NOx, etc. in the exhaust gas discharged from an engine are now strongly requested, even for a small air-cooled two-stroke gasoline engine to be used in a small, portable working machine such as a bush cutter or a chain saw. For example, as seen in the exhaust gas control bill in California, known as CARB TIER2, it is expected that exhaust gas regulations will become increasingly severe.
With a view to complying with such an exhaust gas regulation, the assignee of the present patent application has previously proposed in Japanese Patent Unexamined Publication H9-273416 a muffler having the following construction. The muffler is provided with an expansion chamber in which an exhaust gas ejected from the exhaust port of the engine is introduced. The expansion chamber is separated side by side by means of a partition plate into a first expansion chamber and a second expansion chamber. The partition plate is provided with an exhaust emission purifier, such as an oxidation catalyst, which is composed of a metallic foamed body having gas-permeability. Accordingly, an exhaust gas is introduced initially into the first expansion chamber and then passes through the exhaust emission purifier to the second expansion chamber.
Japanese Patent Unexamined Publicatior. H9-273416 also discloses a muffler wherein, in addition to the above-described construction, the outer wall of the expansion chamber is formed of a double wall consisting of an inner panel and an outer wall panel, which are spaced apart from each other by a suitable distance so as to form an air space therebetween.
Although the muffler proposed in Japanese Patent Unexamined Publication H9-273416 has been shown to significantly reduce THC components contained in an exhaust gas, it has been found by the present inventors that the aforementioned muffler is accompanied with the following problems.
As shown in FIG. 1 of the Japanese Patent Unexamined Publication H9-273416, the expansion chamber of the muffler is generally elongated in the direction orthogonal to the axial direction of the cylinder of the internal combustion engine, and the expansion chamber is separated side by side (laterally) by means of a partition plate into a first expansion chamber and a second expansion chamber. In other words, the expansion chamber is partitioned laterally of the ejecting direction of exhaust gas from the exhaust port of the engine, so that the exhaust gas is radially ejected from the exhaust port into the first expansion chamber. As a result, part of the ejected exhaust gas which is flowing downward cannot be dispersed by the inner wall of the first expansion chamber, thus causing this portion of exhaust gas to directly impinge against an upper portion of the exhaust emission purifier and then to pass through the exhaust emission purifier (oxide catalyst) from the first expansion chamber, while being directed in the same direction as (or in parallel with) the ejecting direction of the exhaust gas, into the second expansion chamber.
As a result, the temperature distribution of the inlet side of the exhaust emission purifier (or the first expansion chamber side) becomes non-uniform, thereby making it impossible to optimize the temperature of this region as a whole for the activation of the purifier. Furthermore, depending on the location of the exhaust emission purifier, not only the outlet temperature of the exhaust gas but also the surface temperature of the muffler may become excessively high due to the heating of the exhaust emission purifier, thus accelerating in the deterioration of the exhaust emission purifier or impairing the durability of the exhaust emission purifier.